Power operated driving apparatuses for fasteners, energized, for example by compressed air, generate substantial driving forces. Therefore, if a fastener is not driven into the workpiece, the fastener may, upon exiting the apparatus, resemble a projectile which may injure the operator or other persons present in the neighborhood. Safety regulations, therefore, prescribe that such projectiles must be avoided. One attempt to eliminate these projectiles is to associate with the muzzle of the driving apparatus a sensor element which only permits an energization or release of the driving apparatus when the muzzle to the driving apparatus has been placed in contact with the workpiece. Accordingly, there have been numerous proposals of construction as to how the sensor is to be designed and in what way it prevents release of the driving apparatus as long as the sensor has not been actuated directly by the workpiece. However, sensors suffer from the disadvantage that they substantially enlarge the muzzle of the driving apparatus thereby rendering difficult the exact positioning of the fastener on the workpiece. Additionally, a sensor is unsuited for work of a type where fabrics or the like are to be tucked, as may be the case in upholsteries.
It has therefore become known to associate a release safety means directly to the release lever of the driving apparatus. In such a structure, the release means is pivotally supported about the release lever of the driving apparatus and includes a spring-biased safety or secondary lever having a downward extension which lies in close contact against the release lever and prevents the actuation thereof. Only when the secondary lever is pivoted may the release lever be actuated (German disclosure letter No. 29 23 987). The pivotal movement of the secondary lever preceding the release of the driving apparatus is to be considered as being disadvantageous from a design standpoint. Also, the safety feature resulting from this type of known release means is only conditionally provided because the release lever and the safety lever possess the same direction of actuation. Therefore, in case of an inadvertent contact by the operator or through a dropping movement of the apparatus and impingement thereof on an article, the release means may be readily overcome and the driving apparatus may be energized. Additionally, such release means cannot be readily incorporated in existing driving apparatus.
It has been suggested also to utilize a release safety means in a compressed air operated fastener driving apparatus. Such safety means include positioning a safety valve in the air passage between the release valve and the control valve. While the release valve is operated by the release lever, the safety valve is actuated by an element pivotally supported above the release lever which is connected to a handling means released by the thumb of the actuating hand. When operating such driving apparatus, the element must first be held against the safety valve through actuation by the thumb before the control valve may be triggered through an actuation of the release valve (German patent letter No. 28 11 339). However, such structures are very costly because of the requirement of an additional valve and are further disadvantageous because of the air consumption by the safety valve, because compressed air escapes through the safety valve when the release lever has already been actuated but the safety valve has not yet been closed. Moreover, the insertion of a safety valve within an existing driving apparatus is not readily accomplished and the thumb operation of such safety means is relatively difficult to accomplish. Accordingly, such safety means are not readily available and have been commercially unacceptable.